This invention relates to a wire dot printer which prints by activating an actuator coil for attracting a print lever and moving a print wire attached thereto and, more particularly, to a circuit for activating the actuator coil within the print head to cause the print lever to move.
In conventional wire dot printer print heads, at least one print lever is movably mounted therein. A print wire is affixed at a distal end of the print lever. An electromagnetic coil provides an attractive force for attracting the lever causing the print wire to strike against the platen. Once printing is completed, the electromagnetic coil may perform a repulsion to repulse the print lever to a return position or a return spring may be mounted within the print head to push the lever to the return direction after printing. Rebounding occurs during the return of the lever to its starting position and, therefore, a damper member made of a highly viscoelastic material, such as rubber, is provided to stop such rebounding.
Such a damper member will sufficiently dampen the rebounding of the wire at room temperature so that the wire and lever will be stopped at the initial starting position, as shown in the solid line of FIG. 6. However, when there is a high print density data to be printed or when the ambient temperature becomes very low, the damper member loses its viscoelasticity. As a result, the rebounding action shown by the wire and lever cannot be sufficiently dampened resulting in the motion illustrated by the dashed line in FIG. 6. Consequently, the print lever which has returned to strike against the damper member during the return movement cannot be sufficiently decelerated so that the wire again moves forward striking the platen performing ghost printing. Additionally, the print wire may be caught on the ribbon during this rebound striking.
Prior art printers which overcome these defects are known from Japanese Laid Open Patent Application No. 61-22761 and Japanese Laid Open Patent Application No. 63-317370 which provide for reenergizing an electromagnetic coil for a short period after completion of the printing to cancel any excessive return or rebound force. However, since the repulsive force acting on the print lever must vary greatly depending on the elasticity of the damper member and, in turn, the elasticity of the damper member varies with the ambient temperature, when the temperature of the print head deviates from an upper and lower predetermined limit, the print wire continues to tend to rebound into an unwanted striking position.
Accordingly, it is desired to provide a print head driving circuit for a wire dot printer which overcomes the shortcomings of the prior art by preventing rebounding of the print wire even when the ambient temperature of the print head falls outside of a predetermined range.